1. Field of the Invention
Metal particles find a wide range of use as fillers, active media, explosives, magnetically sensitive materials, decorative materials, taggants, and reflective material. The present invention relates to the field of metal nanoparticle manufacture and apparatus for the manufacture of nanoparticles.
2. Background of the Art
Many processes are available for the manufacture of small metal particles. These processes cover a wide range of technologies and exhibit a wide range of efficiencies. Some processes produce dry particles, while other processes produce particles in liquid dispersions.
Numerous references have appeared describing use of the gas evaporation technique to produce ultrafine metal powders, especially magnetic metal/metal oxide powders (often referred to as magnetic pigments). These appear to exclusively refer to a dry process and do not involve contact with liquids. Yatsuya et al., Jpn. J. Appl. Phys., 13, 749 (1974), involves evaporation of metals onto a thin film of a hydrocarbon oil (VEROS technique) and is similar to Kimura (supra). Nakatani et al., J. Magn. Magn. Mater., 65, 261 (1987), describe a process in which surface active agents stabilize a dispersion of a ferromagnetic metal (Fe, Co, or Ni) vaporized directly into a hydrocarbon oil to give a ferrofluid using a metal atom technique. The metal atom technique requires high vacuum (pressures less than 10xe2x88x923 torr) such that discrete metal atoms impinge onto the surface of a dispersing medium before the metal atoms have a chance to contact a second species in the gas phase. In this metal atom process, nucleation and particle growth occur in the dispersing medium, not in the gas phase. Thus, particle size is dependent on the dispersing medium and is not easily controlled. Additionally, U.S. Pat. No. 4,576,725 describes a process for making magnetic fluids which involves vaporization of a ferromagnetic metal, adiabatic expansion of the metal vapor and an inert gas through a cooling nozzle to condense the metal and form small metal particles, and impingement of the particles at high velocity onto the surface of a base liquid.
Kimura and Bandow, Bull. Chem. Soc. Japan, 56, 3578 (1983) disclose the nonmechanical dispersing of fine metal particles. This method for prepares colloidal metal dispersions in nonaqueous media also uses a gas evaporation technique. General references by C. Hayashi on ultrafine metal particles and the gas evaporation technique can be found in Physics Today, December 1987, p. 44 and J. Vac. Sci. and Tech., AS, p. 1375 (1987).
EPA 209403 (Toyatoma) describes a process for preparing dry ultrafine particles of organic compounds using a gas evaporation method. The ultrafine particles, having increased hydrophilicity, are taught to be dispersible in aqueous media. Particle sizes obtained are from 500 Angstroms to 4 micrometers. These particles are dispersed by ultrasound to provide mechanical energy that breaks up aggregates, a practice that in itself is known in the art. The resulting dispersions have improved stability towards flocculation.
Other references for dispersing materials that are delivered to a dispersing medium by means of a gas stream include U.S. Pat. No. 1,509,824, which describes introduction of a molecularly dispersed material, generated either by vaporization or atomization, from a pressurized gas stream into a liquid medium such that condensation of the dispersed material occurs in the liquid. Therefore, particle growth occurs in the dispersing medium, not in the gas phase, as described above. Furthermore, the examples given are all materials in their elemental form and all of which have appreciable vapor pressures at room temperature.
U.S. Pat. No. 5,030,669 describes a method consisting essentially of the steps: (a) vaporizing a nonelemental pigment or precursor to a nonelemental pigment in the presence of a nonreactive gas stream to provide ultrafine nonelemental pigment particles or precursor to nonelemental pigment particles; (b) when precursor particles to a nonelemental pigment are present, providing a second gas capable of reacting with the ultrafine precursor particles to a nonelemental pigment and reacting the second gas with the ultrafine precursor particles to a nonelemental pigment to provide ultrafine nonelemental pigment particles; (c) transporting the ultrafine nonelemental pigment particles in said gas stream to a dispersing medium, to provide a dispersion of nonelemental pigment particles in the medium, the particles having an average diameter size of less than 0.1 micrometer; wherein the method takes place in a reactor under subatmospheric pressure in the range of 0.001 to 300 torr.
U.S. Pat. No. 5,106,533 provides a nonaqueous dispersion comprising pigment particles having an average size (diameter) of less than 0.1 micrometer dispersed in an organic medium. That invention provides an aqueous dispersion comprising certain classes of inorganic pigment particles having an average size (diameter) of less than 0.1 micrometer dispersed in a water or water-containing medium. The dispersions require less time for preparation, are more stable, have a more uniform size distribution, a smaller number average particle diameter, fewer surface asperities, and avoid contamination of dispersed material due to the presence of milling media and the wear of mechanical parts, these problems having been noted above for dispersions prepared by conventional methods employing mechanical grinding of particulates. Additionally, no chemical pretreatment of the pigment is required in order to achieve the fine particle sizes obtained in the final dispersion. The pigments of the dispersions are found to have narrower size distributions (standard deviations generally being in the range of xc2x10.5 x, where x is the mean number average particle diameter), are more resistant to flocculation (i.e., the dispersions are stable, that is they are substantially free of settled particles, that is, no more than 10% of the particles settle out for at least 12 hours at 25xc2x0 C.), and demonstrate superior overall stability and color as demonstrated by lack of turbidity, by increased transparency, and by greater tinctorial strength, compared to mechanically dispersed pigment dispersions. Furthermore, the method requires no mechanical energy, such as ultrasound, to break up aggregates. Aggregates do not form since there is no isolation of dry ultrafine pigment particles prior to contacting the dispersing medium. The dispersions of any organic or inorganic pigment or dispersion that can be generated from a pigment precursor, are prepared by a gas evaporation technique which generates ultrafine pigment particles. Bulk pigment is heated under reduced pressure until vaporization occurs. The pigment vaporizes in the presence of a gas stream wherein the gas preferably is inert (nonreactive), although any gas that does not react with the pigment may be used. The ultrafine pigment particles are transported to a liquid dispersing medium by the gas stream and deposited therein by bubbling the gas stream into or impinging the gas stream onto the dispersing medium.
U.S. Pat. No. 6,267,942 describes a process for manufacture of spherical silica particles. Silica gel particles to be dispersed in a mixed solution of an alkali silicate and an acid are required to have an average particle size of from 0.05 to 3.0 micrometers. In a ease where the average particle size of the silica gel particles is smaller than 0.05 micrometers, mechanical strength of the spherical silica particles to be obtained will be low, and irregular particles are likely to form, such being unsuitable. Similarly, in a case where the average particle size of the silica gel particles is larger than 3.0 micrometers, mechanical strength of the spherical silica particles to be obtained will be low, and irregular particles are likely to form, such being unsuitable. The more preferred range of the average particle size of the silica gel particles is from 0.1 to 1.0 micrometers.
A more recent advance in particle coating technology is the use of fluidized bed systems, and in particular, magnetic fluidized bed systems such as that shows in U.S. Pat. No. 5,962,082 (Hendrickson et al.). There, a magnetic field fluidizes a bed of magnetically responsive particles. The magnetically responsive particles and/or other particles carried into a fluidized bed are coated with a material (e.g., a liquid) provided in the fluidized environment. The coating composition may even be transferred from the magnetic particles to non-magnetic particles. This process provides excellent control over the coating thickness, can produce large volumes of coated particles, and provides many other advantages.
U.S. Pat. No. 5,958,329 describes a method and apparatus for producing nanoparticles (there defined as from 1 to 50 nano-meter diameter particles) at a high rate. Two chambers are separated by a narrow duct. A source material is provided from a lower chamber where the source material is heated (e.g., to vaporization and then continuously fed into an upper chamber. In the upper chamber, nanoparticles are nucleated, the nanoparticles being formed when the vapor fed from the lower chamber collides with a gas (inert or reactive) in the upper chamber. A cooled deposit site (e.g., defined as finger 107) collects the particles, which are then scraped from the collection site. The particles are said to move to the collection site in a natural connective flow stream.
U.S. Pat. No. 5,128,081 describes a method of preferential phase separation of aluminum oxide nanocrystalline ceramic material. The nanoparticles are collected on a cold surface (20). Following oxidation of the particles, a vacuum chamber (in which the particles were formed) is evacuated and the oxide particles are collected and consolidated under various atmospheric conditions, such as vacuum and selectively with oxygen and/or air.
The collection process in these particle manufacturing and particle treating processes is cumbersome, inefficient, costly, time-consuming and damaging to the particles. For the collection process, the chamber must be opened and particles scraped from the deposition surface. This requires a long term shut down of the system. Scraping of particles from the deposition surface will fracture some particles and leave others agglomerated. Scraping can also damage the deposition surface. The small elongate finger deposition surface allows for the production and collection of only small amounts of materials layering of collected particles reduces the efficiency of deposition onto the surface. Coating of the particles can be done, but only as re-dispersion of the dried and agglomerated particles.
An alternative method of particle collection is filtration. This is performed by placing in sequence a source of particles, a filtration medium and a vacuum source. The filter has two surfaces, one front surface facing the particle source and the other rear surface facing the vacuum source. The reduced pressure at the rear surface allows the higher pressure at the font surface to push gas and particles against the filter where the particles are entrapped. There are a number of problems in a filtration system, particularly when it is used with nanoparticles. For example, to collect nanoparticles having an average particles diameter of from 1 to 100 nanometers, the largest pore size in the filter must be less than about 1 nanometer. It is difficult to maintain an effective pressure across that filtration surface, even before particles start collecting. As nanoparticles collect on the filter surface, gas flow (and pressure driven movement) become more restricted, fewer particles can collect, and process efficiency diminishes. The particles clog pores rapidly and particles do not collect efficiently.
U.S. Pat. No. 5,857,840 describes a vacuum pump system for making a closed container vacuous, comprising a vacuum pump and a dust collector provided on a pipe connecting the closed container and the vacuum pump, the pipe including:
a main pipe having a first main pipe which connects the closed container and the collector and a second main pipe which connects the centrifugal collector and the vacuum pump;
a bifurcated pipe which is branched out from the first main pipe and connected to the vacuum pump;
a metal mesh dust collector disposed on the bifurcated pipe; and
pipe switching means for switching over between the main pipe and a bifurcated pipe. The dust collector is provided intermediate the source of dust and vacuum pump, which may include a dry pump.
A particle collection system with increased collection efficiency for the collection of nanoparticles comprises a source of particles, a dry pumping system, and a particle collection surface. The position of a dry pumping system in advance of the particle collection surface maintains a particle moving effort, without wetting particles and causing them to agglomerate, and increases collection efficiency.
The placement of the collection units between the nanoparticle source and vacuum pumps causes severe problems in maintaining system vacuum and related high evaporation rates. Wet collection systems are difficult to operate in a vacuum environment; however, the operation of wet collection systems provides slurries in a number of different solvents, which can be posttreated by in-situ polymerization techniques to coat the nanoparticles. The particles in the resulting slurries can be coated with fluoropolymers, such as teflon and polyvinylidene difluoride (PVdF) by in-situ polymerization methods. This differs from earlier work by the use of high pressure reactor technology to provide a teflon or PVdF coating onto the particle. This is the first known application of these polymers in an in-situ polymer coating process.
A source of nanoparticles is provided. The source may be a primary source where particles are being manufactured (e.g., sputtering, spray drying, aerial condensation, aerial polymerization, and the like). The source of nanoparticles may also be a secondary source of particles, where the particles have been previously manufactured and are being separately treated (e.g., coating, surface oxidation, surface etching, and the like). These nanoparticles are provided in a gaseous medium that is of a sufficient gas density to be able to support the particles in flow. That is, there must be sufficient gas that when the gas is moved, the particles will be carried. With nanoparticles (Particles having number average diameters of 1 to 100 nm, preferably 1 to 80 nm, or 1 to 70 nm, and as low as 1 to 50 nm) only a small gas pressure is needed, such as at least 0.25 Torr although higher pressures greater than 0.25 Torr, greater than 0.4 Torr, greater than 0.6 Torr, and greater than 0.75 Torr greater than 0.9 Torr are preferred.
The gas-carrying medium may be or have been reactive with the particles or may have some residual reactive materials in the gas. It is preferred that the gas is relatively inert to the apparatus environment. Gases such as nitrogen, carbon dioxide, air and the like are preferred.
The propulsion system for the gas carrying medium and the nanoparticles is a dry mechanical pumping system for gases. A dry pumping system is used to prevent contamination of the particles by lubricants. These dry pumping systems for gases are well known in the semiconductor industry for conveying air, particulate and vapors without collection occurring in the pump. They are pumping systems that utilize oil-less seals to maintain vacuum conditions at the pump inlet. Examples of such dry pumps and dry vacuum pumps in the literature are found in U.S. Pat. No. 4,452,572 (Robert Evrard) generates a dry vacuum when acting as an additional stage to a conventional vacuum pump. It cites a tubular diaphragm that admits a pressure differential across the diaphragm to allow the diaphragm to conform to the contour of the pumping chamber body and thus expel gas via a top valve. U.S. Pat. No. 5,971,711 describes a control system for pumps, including dry pumps based on a Roots system pump.
U.S. Pat. No. 6,050,787 provides a dry pump comprising a magnetically responsive elastic tube stretched onto, thereby sealing to, a shaft with inlet and outlet ports at or adjacent to it""s ends of the tube. Local to the inlet port a magnetic field is generated in the enclosing body. This field is substantially concentric to the tube, which then responds by expanding circumferentially towards the magnetic field. This creates a volume between the tube and shaft, the length of tube outside the influence of the magnetic field remains sealed upon the shaft. Subsequent movement of the magnetic field along the axis of the pump gives transport of this volume and any media now enclosed within it from the inlet port to the outlet port, whereupon reduction of the magnetic field results in exhaustion of the volume. This cycle results in pumping action.
Other general disclosures of mechanical dry pumps are provided in U.S. Pat. Nos. 6,090,222; 6,161,575; 5,846,062 (which describes a screw type dry vacuum pump having dual shafts is disclosed, whereby the process gas is transported through three compartments, a gas admittance pump section, a central drive motor section, and a gas discharge pump section. By placing the drive motor in the center of the pump, it becomes possible to design a pump having the dual shafts supported only at one end, thus enabling to mount the rotors at the free ends of the pump which are closed with end plates which can be removed easily for servicing the pump sections. Synchronous operation of the dual shaft pump by magnetic coupling enables to lower power consumption and to extend the range of operable pressures.
The collecting medium for the nanoparticles may comprise electrostatic surface collectors, electrostatic filter collectors, porous surfaces (e.g., fused particle surfaces), centrifugal collectors, wet scrubbers, liquid media collectors and physical filter collectors. The liquid media collectors)with subsequent separation of the liquid and the particulates) are more amenable in the practice of the present invention. Also know as wet scrubbers, these liquid collection media are more amenable to this arrangement due to process and safety factors allowing more volatile solvents to be utilized away from the formation chamber for the nanoparticles. Wet scrubbers also provide slurries suitable for post-treatment and polymer coating by in-situ polymerization, particularly in the case of fluoropolymer coatings. Examples of this are Teflon, Polyvinylidene difluoride (PVdF), and their respective copolymers.
The use of the present arrangement of nanoparticle source, dry pump and collector has been found to increase particle collection efficiency by as much as 100% in comparison to the conventional source, filter pump system, even where the same nanoparticle source is present, the same filter and the same pump is used in the different order. The utilization of this arrangement of the pumping scheme may also benefit the collection of the nanoparticles. By injecting low volatility solvents into the inlet of the pump with the nanoparticle loaded gas stream, the dry pump may also be utilized as a wet scrubber with better than 90% collection efficiency. Suitable solvents are the various available Isopar(copyright) media and Purasolv(copyright) media.
Small particles of metals are prepared by an evaporative method with a unique collection method that increases the production efficiency of the process by dramatic degrees. The process comprises evaporating a metal and then providing a mechanical pump that either draws the gas phase metal into a liquid condensation-collection zone or combines a liquid condensation-collection zone within the mechanical pump. The non-metal gaseous material remaining after condensation removal of the metal material is withdrawn from the material stream, while the liquid condensing phase with the condensed metal particles is separated, the liquid condensing phase carrier removed, and the particles collected. As compared to known prior art methods, the use of the intermediate positioned mechanical pump or contemporaneous mechanical pump and condensation-collection zone increases the overall collection/manufacturing efficiency of the process by at least 25%.